In a Wi-Fi® wireless networking environment, it is possible that multiple users simultaneously transmit to an access point using multiple spatial streams. The challenge is how to detect different asynchronous users, perform synchronization and channel estimation of their independent signals, and then accurately decode the symbols for each user.
The IEEE 802.11ac specification defines downlink Multi-User Multiple Input Multiple Output (MU-MIMO) but not uplink MU-MIMO (UL-MU-MIMO). UL-MU-MIMO is a way to “add capacity” by using the spatial resources at a wireless access point to receive multiple parallel transmissions from client devices.